Imaging optical systems are disclosed, for example, in U.S. Pat. No. 6,750,948 B2, US 2006/0232867 A1, EP 0 267 766 A2, U.S. Pat. No. 7,209,286 B2 and WO 2006/069725 A1. WO 2005/098506 A1 discloses, for example, a catadioptric imaging optical system with a minimum ring field radius.